This invention relates to semi-crystalline polyamides and, more particularly, to modified semi-crystalline polyamides.
Semi-crystalline or crystalline polyamides (CPA) are known in the art and are described, for example, in U.S. Pat. Nos. 4,501,844 and 4,806,589. In the '589 patent, CPA is added to polyethylene terephthalate (PET) to increase the crystallization rate of PET. The terms "crystalline" and "semi-crystalline" are used herein interchangeably, although "semi-crystalline" is a more appropriate term to describe crystallinity in polymers than "crystalline" because most "crystalline" polymers have an appreciable amount of amorphous, uncrystallizable material even after their level of crystallinity has been maximized. Semi-crystalline polyamides are known to be useful in applications that require good load bearing, good solvent resistance and good heat resistance, such as automotive under the hood applications or electronic parts.
To achieve maximum heat resistance, semi-crystalline polyamides (CPA) need high degree of crystallization. Insufficiently crystallized CPA parts can distort severely when heated above their T.sub.g (about 125.degree. C.), whereas CPA parts with sufficient degree of crystallization do not distort significantly until temperatures approach the melting point (T.sub.m) which is about 285.degree. C. However, highly crystallized crystalline polyamides are brittle and, therefore, have limited utility. To minimize the effects of brittleness in highly crystallized CPAs, fillers and fibrous reinforcements are usually used. Using fillers and reinforcements, however, do not provide a satisfactory solution to the problem because they cannot be used in applications which require unfilled compositions. One such application is for lamp reflectors which require a smoother surface than can be achieved with filled compositions.
Typically, the mold temperature range required to achieve sufficient crystallization with CPA during injection-molding cycle of less than about 1 minute is between about 150.degree. C. and about 175.degree. C. If the mold temperature is below about 150.degree. C., parts with an amorphous skin and a crystalline core are formed. The amorphous skin can be detected visually because amorphous CPA is transparent and crystallized CPA is relatively opaque. CPA with sufficient degree of crystallization will be referred to hereinafter as "fully crystallized CPA" and that with insufficient degree of crystallization will be referred to hereinafter as "partially crystallized CPA."
It would be desirable to provide fully crystallized crystalline polyamides which do not contain reinforcements or fillers, yet have good toughness and good thermal properties.